1. Field of the Invention
The present invention relates to aqueous dispersions containing polyisocyanates in powdered form and isocyanate-reactive polymers, to adhesive compositions containing these dispersions and to a process for bonding substrates with these compositions, which crosslink at room temperature.
2. Description of the Prior Art
One-component (1K) polyurethane dispersions, which are used for heat-activated bonds on the basis of their linear polymer chain that crystallizes at temperatures below 100xc2x0 C., preferably below 60xc2x0 C., give low 1K thermal stabilities without additional isocyanate crosslinking. For that reason they are usually processed in combination with liquid polyisocyanates which are emulsifiable in the aqueous dispersion, for example Desmodur DA polyisocyanate (Bayer AG). Coatings prepared from these two-component (1K) compositions possess markedly better thermal stability values. For example, in the bonding of an SBR rubber (SBR vulcanate (Nora test), PFIxe2x80x94Prxc3xcf- und Forschungsinstitut fxc3xcr die Schuhindustriexe2x80x94Pirmasens), commercial polyurethane dispersions for heat-activated adhesives give a softening point of approximately 65xc2x0 C. without isocyanate crosslinking, but when crosslinked with Desmodur DA polyisocyanate, the softening point is approximately 130xc2x0 C. However, that gain is only achieved with the cost of a higher technical outlay for 2K processing, in conjunction with possible sources of error. This is because for each individual batch it is necessary to disperse the isocyanate crosslinking agent, which is to be prepared separately, homogeneously and precisely in terms of quantity.
Although the storage stability of those mixtures is sufficient to allow the adhesive to be processed without difficulty over a period of one day, it is not sufficient to allow the crosslinking agent to be incorporated into the adhesive formulation by the adhesive manufacturer.
1K reaction compositions with pulverulent polyisocyanates, in which the isocyanates are incorporated into the formulation without previously being deactivated at the surface, are known. DE-A 1 570 548 describes storable, heat-curable polyurethane compositions with powdered uretdione isocyanates which have not undergone surface deactivation. They can be stored at room temperature for a period of from several weeks to a few months and must be cured by heating for 30 to 90 minutes at temperatures of 100 to 160xc2x0 C. However, DE-A 1 570 548 does not describe the use of pulverulent, non-deactivated solid isocyanates in aqueous polymer dispersions.
EP-A 0 467 168 claims compositions containing surface-deactivated aqueous polyisocyanate solid suspensions and polymer dispersions having NCO-reactive groups for flexible coatings for textiles substrates. The compositions have a long storage life. The coatings are post-crosslinked at elevated temperature at the same time as they are dried. The composition is produced by mixing the polymer dispersion with the dispersion of the surface-deactivated polyisocyanate. The dispersion and surface deactivation of the polyisocyanate are described in EP-A 0 204 970, which requires a separate operation prior to incorporation into the polymer dispersion.
EP-A 0 922 720 describes the use of aqueous dispersions which contain at least one surface-deactivated solid polyisocyanate and at least one isocyanate-reactive polymer for the production of coatings or powders which are storage stable at room temperature, have latent reactivity and are crosslinked by heating. The preparation of the polyisocyanate dispersion and the surface deactivation are also carried out in accordance with EP-A 0 204 970 as a separate operation prior to incorporation into the dispersion. Although deactivation of the isocyanate could result in a relatively good storage life of the dried polymer film, it does require the additional processing steps.
The search for aqueous adhesive dispersion systems, which are in one-component form for final processing, i.e., they can be processed without the addition of further crosslinking agents, and which crosslink after application without additional measures to form heat-resistant bonds, has for a long time been an intensively researched field.
Accordingly, an object of the present invention is to provide a one-component adhesive composition that has an adequate pot life is self-crosslinking without special after-treatment.
This object may be achieved with the one-component, storage stable compositions according to the invention which contain finely divided polyisocyanate powders and aqueous isocyanate-reactive polymer dispersions and which may be crosslinked at room temperature.
The present invention relates to an aqueous dispersion containing an isocyanate-reactive polymer and a polyisocyanate, wherein the polyisocyanate has a melting or softening point of  greater than 30xc2x0 C. and is in the form of a finely divided powder having a mean particle diameter of less than 100 xcexcm.
The present invention also relates to an adhesive composition containing this dispersion and to a process for bonding substrates with this composition.
The storage stable compositions according to the invention are produced by mixing finely divided polyisocyanate powders or corresponding pastes in inert carrier liquids with aqueous isocyanate-reactive polyurethane dispersions. In accordance with the present invention the polyisocyanates include diisocyanates.
A valuable property of the compositions according to the invention results from the fact that, contrary to known systems, the finely divided solid polyisocyanates are mixed into the aqueous dispersions directly, i.e., without deactivation or encapsulation (see Example 1). Compositions are obtained which surprisingly and unexpectedly can be stored at conventional storage temperatures of 20xc2x0 to 30xc2x0 C. without substantial losses in the isocyanate content (see Example 2).
The adhesive bonds produced using such compositions crosslink even at room temperature over a period of several days, even if no additional tempering is carried out. To the contrary in the examples of EP-A 0 922 720 when a deactivated solid isocyanate is used, an additional tempering step at elevated temperature is required for the crosslinking (see Examples 3.1 and 3.2). Also, in contrast to EP-A 0 922 720, no catalysts are required for accelerating the reaction of the isocyanates. In polyurethane dispersions in particular, which generally contain polyester polyols as the base raw materials, the use of metal catalysts would be very disadvantageous with regard to the hydrolytic stability of the polymer chain.
Crosslinking of the compositions according to the invention leads to a considerably increased softening point and hydrolytic stability of the bond. In addition, in comparison with liquid isocyanate crosslinking agents, such as Desmodur DA polyisocyanates, and especially in comparison with deactivated solid isocyanates, markedly increased strengths are found in the peel test on plasticized PVC.
It is of particular importance that when using polyurethane dispersions prepared on the basis of crystallizing polymer chains, the adhesive films can be stored for several days or weeks at room temperature prior to the heat-activated bonding that is required in the case of such polymers (i.e., bonding by decrystallization of the polymer chain) without the heat activatability being impaired by the onset of crosslinking. However, after the heat-activated bonding has been carried out using the correct activation conditions (see Example 3.1.1), the crosslinking reaction proceeds at room temperature within several days without difficulty.
By using the compositions according to the invention based on non-deactivated solid isocyanates in polymer dispersions, the additional work involved in tempering the bond and the risk of hydrolysis of the polymer chain by catalysts, which are optionally used, are therefore avoided.
Although the compositions according to the invention exhibit reduced storage stability of the dried adhesive film as compared with the compositions described in EP-A 0 922 720, that is not a relevant problem in terms of application because the period of time until the onset of crosslinking that impedes adhesiveness is still totally sufficient even without deactivation of the polyisocyanate surface (see Examples 4.1 and 4.2).
For the production of the compositions according to the invention, the pulverulent polyisocyanate, optionally adjusted to the desired particle size limit, for example by sieving, is mixed into the polymer dispersion. In order to simplify the milling process and avoid the formation of dust, the powder may be made into a paste in an inert carrier liquid. The process simplifies previously known processes since the separate preparation of a dispersion of the xe2x80x9cdeactivatedxe2x80x9d solid isocyanate is not required. In particular, the excess enveloping amine in the dispersion of the deactivated solid isocyanate, which has a negative effect on colloidal chemical stability, is avoided. The excess enveloping amine would have to be removed from the aqueous phase by filtering off the deactivated solid. That complex preparation step is not necessary in the case of the use according to the invention of the non-deactivated solid isocyanates.
Production is optionally carried out in combination with additives such as protective colloids, emulsifiers, thickeners, solvents, plasticizers, pigments, fillers, dyes and other known additives employed in the preparation of adhesive dispersions and coatings.
The polyisocyanates preferably have melting or softening points of  greater than 30xc2x0 C. and are mixed into the aqueous dispersions in the form of finely divided powders or in the form of pastes in inert carrier liquids having a mean particle size of substantially less than 100 xcexcm, preferably less than 50 xcexcm.
The viscosity of the dispersions is adjusted to at least 500 mPas, preferably to at least 2000 mPas.
Preferred polymer dispersions are isocyanate-reactive polyurethane or polyurea dispersions as well as polymers of 2-chlorobutadiene.
Special preference is given to dispersions of isocyanate-reactive polyurethanes which are composed of crystallized polymer chains which, when measured by means of thermomechanical analysis, decrystallize at least partially at temperatures of less than 100xc2x0 C., preferably less than 90xc2x0 C. and more preferably less than 65xc2x0 C.
Preferred polyisocyanates include dimerization products, trimerization products and urea derivatives of TDI or IPDI.
The dispersions according to the invention are preferably used for the production of coatings or adhesive bonds which, after drying, crosslink over a period of a few days without the additional supply of heat. They have a markedly improved softening point, fastness to water and resistance to solvents.
In order to produce such adhesive bonds, the dried adhesive layer on the substrate to be bonded is decrystallized by heating for a short time, preferably at a temperature of less than 110xc2x0 C., and joined in the decrystallized state. The adhesive layer may also be dried on a substrate and then pressed with a foil material which has been softened plastically by heating, so that the foil in contact with the adhesive film assumes a temperature above the decrystallization temperature of the polymer in the adhesive film as described above.